The present invention relates to a silicon carbide semi-conductor device such as a metal oxide semiconductor field effect transistor (MOSFET), a metal semiconductor field effect transistor (MESFET), a bipolar transistor, and a vertical type of MOS transistor using a silicon carbide substrate and a method of producing the same. More particularly, the present invention relates to a silicon carbide semiconductor device using a metal nitride as a contact electrode and a method of producing the same.
A silicon carbide (SiC) semiconductor has a wide forbidden band (2.2 to 3.3 eV) compared to other semiconductors such as silicon (Si) and gallium arsenide (GaAs) which are widely used in practice. Also, the silicon carbide semiconductor is stable in thermal, chemical and mechanical property and has superior resistance of a radiation ray. Therefore, a semiconductor device using silicon carbide can be used as a device having high reliability and stability under severe condition of high temperature, high power and the radiation ray under which a semiconductor device composed of other material cannot be used. Specifically, application to a light emitting device emitting blue light which is in conjunction with the wide forbidden band enters the stage of practical use.
However, application of silicon carbide to electronic devices is not sufficient compared to the application to the light emitting device. One of the reasons is that there are not yet developed an electrode material suitable for a complicated electronic device producing process and having electrically good ohmic contact and the forming method of the electrode.
In research, it is tungsten (W), titanium (Ti), nickel (Ni) and silicide of these metals that is researched as electrode material for forming ohmic contact with the N-type silicon carbide semiconductor. The resistivities of these contacts are about 10.sup.-1 to 10.sup.-4 ohm.cm.sup.2. These values are greatly different from those practically used in the Si and GaAs (about 10.sup.-6 ohm.cm.sup.2).
W, Ti and silicide thereof are degraded in contact characteristic if heat treatment is performed at a low temperature. Therefore, heat treatment of a high temperature of 1100.degree. C. or above is required to obtain a low contact resistivity even in a case of Ni which is said to have low contact resistivity. In this manner, the above materials are not suitable for electrode material. Also, TiN is disclosed as electrode material having ohmic contact to N-type silicon carbide which does not require high temperature heat treatment (R. C. Glass et al., "Low energy ion-assisted deposition of titanium nitride ohmic contacts on alpha (6 H)--silicon carbide", (Appl. Phys,. Lett 59 (22), pp. 2868-2870 (1991)). According to this paper, the growth of TiN film is performed by a vapor deposition method of Ti in which nitrogen ions are assisted by an ion gun.
On the other hand, a MOSFET is disclosed in, for example, Unexamined Published Japanese Patent Application 60-142568 as a conventional semiconductor device using silicon carbide substrate. In the semiconductor device, after source/drain regions are formed in a P-type silicon carbide single crystal substrate, a Ni layer is formed on the source/drain regions as ohmic contacts electrode and an aluminium (Al) layer is used for a gate electrode. Metal wirings are connected to these electrodes. Also, platinum (Pt), gold (Au), and aluminium (Al) are used for the gate electrode in a conventional MESFET device.
However, in a TiN electrode which is formed by nitrogen ion assisted vapor deposition of Ti, there is a problem in that a contact resistivity cannot be reduced because nitrogen is not introduced in a surface layer portion of a SiC region having a predetermined thickness, the portion contacting the TiN electrode, or nitrogen is not electrically activated even if the nitrogen is introduced. In order to solve the problem, a method is interested in which nitrogen as an N-type dopant to silicon carbide is introduced in the SiC region to electrically activate the N-type carriers. For this purpose, heat treatment is performed after the nitrogen ion implantation. In this method, however, it is difficult to introduce nitrogen ions only into the surface layer portion of SiC region with a high concentration and therefore there is a problem in that it is not easy to form a fine contact.
Otherwise, in a semiconductor device disclosed in Unexamined Published Japanese Patent Application 60-142568 there are the following problems. That is, first, if Ni is used as an ohmic contact electrode material in the source/drain region, the contact resistivity is still greater than that of semiconductor device of Si and GaAs, as described above. Second, in a case that Al is used for the gate electrode material, Al is melt because of a high temperature heat treatment. Therefore, a refractory metal such as molybdenum (Mo) and tungsten (W) needs to be used in actual. When such a refractory metal is used, the metal reacts with Al or tungsten silicide (WSi.sub.x) of the interconnection formed on the metal, which results in troubles such as generation of an irregular portion and holes, and peeling of the gate electrode due to the reaction resultant product.
The inventors found out that the contact resistivity can be reduced when a metal nitride comprising either one of titanium nitride (TiN), zirconium nitride (ZrN), hafnium nitride (HfN), Vanadium nitride (VN) and tantalum nitride (TaN) is used as the electrode material of SiC region and a nitrogen-rich layer is formed on the surface layer portion of the SiC region on which the metal nitride layer is to be formed. Also, the inventors found out that it can be prevented for the gate electrode material to react with the interconnection by interposing the metal nitride layer between the gate electrode and the interconnection composed of material such as Mo and the WSi.sub.x.